Preamplifier



E .Z 10 12) 13 a Dec. 29, 1953 w. D. MOEHRING ET AL 2,664,469

PREAMPLIFIR Filed March 22, 1951 Patented Dec. 29, 1953 PREAMPLIFIER William D. Moehring, Charlotte, and Lawrence V. Wells, Kalamazoo, Mich.. assignors to Wilcox-Gay Corporation, Charlotte, Mich., a corporation of Michigan Application March 22, 1951, SerialY No. 217,602

4 Claims. (Cl. 17a-100.11)

Our invention relates to phonograph and microphone amplification circuits in radio receiver circuits, and more particularly to circuit arrangements which enable a single tube to be used either as an audio frequency pre-amplifier r as a radio frequency amplifier in a conventional radio receiver circuit. More specifically, `our present invention is a continuation of the invention described in application Serial No.

In radio phonograph combination equipment,

lthe audio stage of the radio receiver circuit is 'usually employed in reproduction from phonograph recordings or for the amplification of microphone signals for public address to be reproduced by the loudspeaker of the equipment.

Although the audio frequency stage in the conventional radio receiver amples the signal sufficiently for good loudspeaker operation, it

'has been found inadequate for reproduction ment where the microphone pre-amplifier represents not only increased bulk and cost but also l adds to the power consumption and resultant "heatdissipation of the unit. the-heat dissipation requires increased ventila- Ltion'or, in the alternative, higher operating tem- The increase in peratures must be tolerated. This is necessarily "a design limitation where compactness is a prime consideration.

We have discovered that by the use of suitable means, we can employ one of the tubes in the radio stage as a part of the audio frequency amplier when the system is to be operated for public address or phonograph reproduction.

-Accordingly, an object of the present inven- -tion is to provide novel circuit arrangements in which one of the tubes in the system serves -either as a microphone amplier stage or as aV conventional radio frequency amplifier, depend- "ing on whether the signals are of audio fre- -quency or of radio frequency, respectively.

A further object of my invention is to provide novel circuit arrangements for obtaining dual 'function of the components of one tube of a ral dio phonograph microphone receiving circuit. Still a further object of my invention is to 2 provide a microphone amplifier which does not entail additional heat dissipation beyond that existing when the unit is employed as a conventional radio receiver.

In one of the embodiments of our present invention the use of a single tube as radio frequency amplifier and as audio frequency amplifier when it is to be operated for public address or phonograph reproduction is made possible by the novel introduction of switching means. In switching, however, a radio frequency stage, to function as a pre-amplifier stage, any appreciable change in the capacity coupling between the signal grid and the output plate would render the radio frequency amplifier unstable.

It is accordingly essential in such switching operations to avoid any appreciable change in such coupling.

Accordingly, a further object of our invention is to provide circuit switching means permitting the use of a single tube as either a microphone or as a conventional radio frequency amplifier without significant increase in the signal grid to output plate capacity coupling when the tube is used for radio frequency amplification.

In another embodiment of our present invention the use of a single tube as radio frequency amplifier and as audio frequency amplifier is made possible by the introduction, in the input and output circuit of the tube, of appropriate radio frequency chokes, which, as is known in the art, present high impedance to radio frequency signals and are practically short circuits for audio frequency signals.

Accordingly, another object of our present invention is to provide electric elements in the tubes circuits, that block radio frequency signal while providing paths of low impedance to audio frequency signals.

These and other objects of our invention will be apparent in the description which follows in comiection with the drawings in which:

Figure 1 is a circuit diagram of one embodiment of our invention.

Figure 2 is a circuit diagram of another embodiment of our invention.

Referring now to Figure 1, tube l serves alternatively as a microphone pre-amplifier or as a conventional intermediate frequency amplifier. Switches 2, 3 and mounted on a common operating shaft, are shown in the drawing in the position wherein tube l functions as an intermediate or radio frequency amplifier.

The signal input to the grid 5 of the tube I 1s obtained from the secondary winding 'i of the intermediate frequency transformer t. The tube is biased by means of the cathode resistor 9. Plate I is connected to the primary winding l2 of the intermediate frequency transformer i3. The secondary winding Ill of this transformer I3 is connected to the anode of diode I5 and to the diode filter comprising resistor I'I and condensers I8 and IS.

The diode filter in turn is connected to volume control resistor which precedes the conventional audio or recording amplifier here illustrated in a box diagram since any well known audio arn-V plier stage may be employed;

Automatic volume control bias for grid 5 of tube I is supplied through the filter comprising resistor 22 and condenser 23. The plate and screen supply for tube I is obtained from the high volt-v age source through the resistor 2li Icy-passed to ground through condenser 25. Resistor 2B and condenser 25 form a decoupling filter for radio frequencies in a manner well known in the art.k

In the position shown for switches 2, 3 and il, the microphone 2'! is short-circuited by the switch 2. The equipment `is in condition for reception of radio signals which are received over the box identified as Preceding Portion of Radio Receiver which may be of any conventional construction. Tube I which is a part of the radio ren ceiver in the condition shown in the drawing receives signals through the transformer 3 iinpressed across the input at grid Output signals from the radio frequency stage after detection are impressed across the usual audio frequency amplifier and thence to the loudspeaker in the manner well known in the art.

If now the microphone 2l or phonograph pickup is to be operated, the gang switches 2, t and i are rotated on their shaft in a counterclockwise direction as Viewed from the drawing to their alternate contact positions. As a result of the operation of switch 2 to disengage the contact shown in the drawing to its alternate position, the microphone 2i is coupled through condenser 23 and secondary winding l to the grid 5 of tube i.

^ inasmuch as the transformer 8 is a radio frequency transformer, the secondary winding` 'i has negligible impedance at audio frequency and accordingly will function only as a conductor. As a result of the switching operation at switch 3, re-

sistor 22 which previously was a part of the filter 'circuit including condenser 23 now serves as a grid leak for tube I;

Inasmuch as transformer I3 is a radio frequency transformer primary winding I2 has negligible im- -pedance at audio frequency and therefore effec- `tively connects plate IEl and screen I l so that the tube I now functions as a triode. The majority of pentodes suitable for use as intermediate frequencyl amplifiers provide satisfactory triode operation when the screen and plate are connected together.

Resistor 265 functions as a plate load resistance. It may be of suificientlylow value to maintain good intermediate frequency performance when tube I is used as a part of the radio frequency circuit and, at the same time, be sufficiently large to provide good amplification at audio frequency.

- This'is because the triode connection provides a medium amplification constant with a relatively low plate resistance. Condenser 25, which previously functioned as a radio frequency byepass becomes an audio frequency coupling condenser to volume control 2t over the `switch '4 in its second position.

It will be noted particularly that switches 2, 3 and l are so arranged that the radio frequency connections such as the lead between winding 'I and grid 5, and the lead between plate Ill and winding l2 are not affected in the switching operation. By leaving transformer windings 'I and I2 permanently connected in their respective circuits, the switches do not contribute any capacity coupling which would disturb the normal function of the tube as an intermediate frequency amplier.

Had the switches been inserted directly in the leads of grid 5 and plate i0 as would normally be expected in changing the function of tube I, additional capacity would have been introduced between these elements which would have contributed to amplifier instability or would have reduced the amount of stable gain which could be realized.

It is good engineering practice to disable the preceding portion of the radio receiver when the microphone is connected for use. If this is not done, strong radio signals may be rectified and mixed with the signal from the microphone.

Automatic volume control voltage for the preceding stage or stages of the radio receiver is preferably applied through a separate filter network from point 2S rather than through the filter comprising resistor 22 and condenser 23. This prevents excessive loading of microphone 21 which will ordinarily be a high impedance type unit, such as a crystal microphone. It also prevents hum coupling from the antenna or loop of the radio receiver to grid 5.

Switches 2 and 3 must provide low impedance ground returns when the tube is used as an intermediate frequency ampiifier. 1f a common lead to ground is used, the common impedance between the input and output circuits may contribute to amplier instability. A separate ground lead for each switch is preferred.

To further illustrate the operation of the invention, the following circuit values have been employed in association with a type 6BA6 tube:

9 ohms" 68 n22 megohms 2.2 23 microfarad" v .05 2li ohms 10,000 25 microfarad .01'

Obviously the invention is not restricted to the intermediate frequency radio frequency amplifier shown, but is equally applicable to radio frequency amplifiers functioning at the original signal frequency as well.

Referring now to `Figure 2, which shows the circuit of another embodiment of our invention. tube itil serves alternatively as a microphone. pre-amplifier or as a conventional intermediate frequency amplifier. Y Y

Signals from the Preceding Portion of Radio Receiver, which may be of any conventional` construction, are applied to the primary Winding HB5 of coupling transformer |06. Secondary winding ll of transformer |06, in parallel with capacitor let, constitutes a tuned circuit for the input of tube |30.

IThe signals are then fed to the control grid H0 of tube Illi. The tube I is biased by means of the cathode resistor III. Plate II2 is connected to the tuned circuit H3, consisting of capacitor II@ and primary winding II5 of coupling transformer llt. The secondary winding i il of transformer lit is connected to a capacitor lit, thus forming another tuned circuit for the input of the Following Portion of Radio Receiver, which may also be of any conventional construction.

The plate and screen supply for tube is obtained from the high Voltage source |24 through the resistor |25, 'oy-passed to ground through capacitor |26. Resistor |25 and capacitor |26 form a decoupling filter for radio frequencies in a manner well known.

A microphone |21 or phonograph pickup is connected through coupling capacitor |28 and radio frequency choke |29 to the control grid 0 of tube |00. The circuit consisting of microphone |21, coupling capacitor V|28 and radio frequency choke |29 is by-passed to ground for radio frequency signals by the small radio frequency by-pass capacitor |30. Connected between control grid ||0 and ground is grid leak resistor |35.

An "Audio and/or Recording Amplifier is connected through coupling capacitor |40 and radio frequency choke 4| to the output circuit of tube |00, or more precisely, at point |42 between plate tuned circuit H3 and plate supply resistor |25, so that when tube |00 is to operate as a triode (audio operation), resistor |25 functions as a plate load resistor.

When tube |00 is used as a radio frequency amplifier, signals from the Preceding Portion of Radio Receiver and having a frequency around the anti-resonant frequency of tuned circuit |01, |08, are first amplified by tube |00 andthen through transformer coupling ||6 are fed into the Following Portion of Radio Receiver. These radio frequency signals cannot go into the audio circuits connected to tube |00, since radio frequency chokes |29 and |4| present a path of very high impedance to radio frequency signals. .,Therefore, in radio frequency operations tube |00 operates as a pentode.

When, on the other hand, tube |00 is used as an audio amplifier, audio signals from microphone |21 are fed to the grid ||0 of tube |00 through coupling capacitor |28 and radio frequency choke |29, which acts as a low resistance path since the signals are of audio frequency.

Inasmuch as the transformer |06 is a radio frequency transformer, the secondary winding |01 has negligible impedance at audio frequency and accordingly will function only as a conductor, and resistor operates as a grid leak resistor for grid I0 of tube |00.

Inasmuch as transformer I I0 is a radio frequency transformer, primary winding ||5 has negligible impedance at audio frequency and therefore effectively connects Vplate ||2 and screen grid |50 so that the tube |00 now functions as a triode. The majority of pentodes suitable for use as intermediate frequency amplifiers provide satisfactory triode operation when the screen and plate are connected together.

As above mentioned, resistor |25 functions as a plate load resistance. It may be of sufficiently low value to maintain good intermediate frequency performance when tube |00 is used as a part of the radio frequency circuit and, at the same time, be sufficiently low to provide good amplification at audio frequency.

An Audio and/or Recording Amplifier is connected to the plate load resistor |25 through coupling capacitor l and radio frequency choke 4|, which also functions as a virtual short circuit for signals of audio frequencies.

To further illustrate the operations of our invention, the following circuit values have been employed in association with a type 6BA6 tube:

As above pointed out, obviously our invention is not restricted to the intermediate frequency. radio frequency amplifier shown, but is equally applicable to radio frequency amplifier functioning at the original signal frequency as well.

`Although we have shown a preferred embodiment of our invention, it will be obvious that it may take other forms as herein suggested, without departing from the spirit of the invention. Therefore, we prefer to be limited not by the specific disclosures herein contained, but only by the appended claims.

We claim:

l. In an amplifier circuit having a radio frequency amplifier including at least one stage of radio frequency amplification comprising a pentode having an input and output circuit, a radio frequency transformer in said radio frequency stage connected to the input of said pentode, a second radio frequency transformer in said radio frequency stage connected to the output of said pentode and connecting said anode and screen grid to form a low impedance audio frequency connection therebetween to eifectivelymake said pentode a triode, a microphone connected to the input circuit of said pentode, a first switch having a plurality of positions in one of which it shunts said microphone, a filter including a resistor and a condenser connected between the input and output of said pentode for providing automatic volume control bias for the input of said pentode, a second switch having a plurality of positions, a source of high voltage for said pentode, a resistor connected in the circuit of said high voltage source, a radio frequency by-pass condenser providing a by-passing circuit around said resistor, said second switch in one of its positions completing the circuit of said by-pass condenser, an audio frequency amplifier, a third switch having a plurality of positions in one of which it connects the secondary of said second transformer to the input of said audio frequency amplifier, said microphone connection to said pentode input being operative when said first switch is in another than said one position, circuit connections including said second switch in another than said one of its positions for connecting said filter resistance to the input of said pentode as a grid resistance for said electron tube and circuit connections including said third switch in another than said one position for directly connecting the output of said triode through said by-pass condenser as a coupling condenser to the input of said audio frequency amplier.

2. In an amplifier circuit having an audio frequency amplifier and a radio frequency amplifier including at least one stage of radio frequency amplification comprising a pentode having an input and output circuit, a radio frequency transformer in said radio frequency stage connected to the input of said pentode, a second radio frequency transformer in said radio frequency stage connected to the output of said pentode, a microphone connected to the input circuit of said pentode, a. rst switch having at least a iirst posi-f tion which shunts said microphone and a second position which opens said shunt, a second switch having a rst and second position, a source of high voltage for said pentode, circuit connections including said second switch in its first position for providing a radio frequency condenser bypass from said high voltage source and in its second position for providing a grid resistance for said tube, a third switch having a first and second position, circuit connections including said third switch in its rst position for connecting the secondary of said second transformer to said audio frequency amplifier, and including said second position for connecting the output of said triode directly through said radio frequency condenser by-'pass acting as a coupling condenser to the input o f said audio frequency amplifier.

` 3. In an amplier circuit having an audio frequency amplier and a radio frequency amplifier including at least one stage of radio frequency amplication comprising a pentode having an input and output circuit, a radio frequency transformer in said radio frequency stage connected to the input of said pentode, a second radio frequency transformer in said radio frequency stage connected to the output of said pentode, an audio frequency generating means, a plurality of switches,` circuit connections including one of said .switches for selectively rendering said audio frequency generating means nonoperative and for roperatively connecting said means to the input of said tube, circuit connections including another of said switches for selectively yswitching the grid'return circuit and disconnecting and connecting grid bias to said tube and circuit connections including still another of said switches for selectively connecting the secondary o f said second transformer and the primary of said second transformer to the input 0f Said audiO frequency amplifier.

4, 1n an amplifier circuit having at least one stage o f radio frequency amplification including a pentode having an input and output circuit, a radio frequency transformer in said radio frequency stage connected to the input of said pentode, a secondV radio frequency transformer in said 'radio frequency stage connected to the output of said pentode, a'microphone having circuit connections to the input of said'tube, a switch having a plurality'of positions in one of which it shunts said microphone, a filter including a resistor and a condenser connected between the input and output of said pentode for providing automatic volume control bias for the input of said multi-grid tube, a second switch having a plurality of positions, a source of high voltagel for said pentode, a resistor connected in vthe circuit of said high voltage source, a radio frequency bypass condenser, said second switch in'one of its positions completing the circuit of said by-pass condenser, a third switch having a plurality of positions in one of which it connects the secondary of said second transformer to said audio frequency ampliiier, said circuit connection including said second switch in another ofits positions for connecting said lter resistance as a grid resistor for said triode and circuit connections including said third switch for connecting the output of said triode through said radio frequency icy-pass condenser acting as an audio frequency coupling condenser to the input of said audio frequency ampliiier.

WILLIAM D. MOEHRING. LAWRENCE V. WELLS.

Name Date Moir Aug. 15, 1950 Hayes Apr. 10, 1951 Number 

